This application relates to non-toxic organotin stabilizers. More particularly, this invention relates to organotin stabilizer compositions that contain at least one mono-n-octyltin mercaptoacetic acid ester for halogen-containing polymers that are sufficiently low in toxicity to be suitable for use in packaging materials for food and beverages.
Polymer compositions derived from halogen-containing monomers such as vinyl chloride and vinylidene chlorine are unstable when exposed to temperatures above about 100.degree. C. for the time periods required to process these polymers into shaped articles such as films and containers. A variety of compounds has been developed which effectively prevent or delay the discoloration and embrittlement that attend vinyl chloride polymers processed at these temperatures. Some of these stabilizers are of low toxicity, a discovery that has facilitated the use of vinyl chloride polymers as food packaging materials; such use may take the form of films, bottles, trays and containers.
Organotin compounds containing sulfur have long been recognized as effective stabilizers for vinyl halide homopolymers and copolymers. U.S. Pat. No. 2,789,963 discloses organotin compounds suitable for this purpose. Diorganotin dimercaptides and bis(mercapto-carboxylic acid esters) are said to be particularly preferred, because of their superior performance. However, a shortcoming of many of these sulfur-containing organotin compounds, which can be represented by the general formulae R.sub.2 Sn(SR').sub.2 and R.sub.2 Sn(SR"COOR').sub.2, wherein R and R' represent monovalent hydrocarbon radicals and R" represents a divalent hydrocarbon radical, is that they are too toxic for use in food packaging materials.
The use of di-n-octyltin-S,S'-bis(isoctyl mercaptoacetate) as a food grade stabilizer for vinyl chloride polymers is disclosed in U.S. Pat. No. 3,640,947. This patent teaches that the stabilizer is prepared from a di-n-octyltin oxide composition containing not more than 5% by weight of an n-octylstannoic acid, the precursor of mono-n-octyltin-S,S',S"-tris(isooctyl mercaptoacetate). It was believed necessary to limit the concentration of mono-and/or tri-n-octyltin compounds to less than 5%. A related U.S. Pat. No. 3,390,159, discloses a process for preparing di-n-octyltin oxide from di-n-octyltin dichloride in the absence of impurities. These impurities result from the presence, in the crude di-n-octyltin chloride, of mono-n-octyltin trichloride and tri-n-octyltin chloride in addition to inorganic compounds. One reason for excluding mono- and tri-n-octyltin compounds is that they were believed to increase the toxicity of the resultant di-n-octyltin oxide, which is subsequently reacted with an octyl ester of mercaptoacetic acid to obtain the food grade stabilizers.
At present only di-n-octyltin-S,S'-bis(isooctyl mercaptoacetate) and di-n-octyltin maleate polymer have been recognized by the U.S. Food and Drug Administration (FDA) to be sufficiently low in chronic toxicity and extractability from vinyl halide polymers to qualify as a food-grade stabilizer. The FDA specifications for di-n-octyltin-S,S'-bis (isooctyl mercaptoacetate) require that it be prepared from di-n-octyltin dichloride and that the combined concentrations of mono-n-octyltin trichloride and tri-n-octyltin chloride do not exceed 5% by weight of total organotin halides. The di-n-octyltin dichloride is optionally converted to di-n-octyltin oxide before being reacted with isooctyl mercaptoacetate.
Dialkyltin dichlorides, such as di-n-octyltin dichloride, are conventionally prepared by reacting the corresponding tetraalkyltin compound, such as tetra-n-octyltin, with stannic chloride using equimolar amounts of the two reagents. However, the reaction of a tetraalkyltin compound with stannic chloride produces equilibrium mixtures containing substantial amounts of mono-, and triorganotin chloride, requiring considerable purification to reduce the mono- and tri- species to less than 5%.
The toxicity of a given tin compound cannot be readily predicted. That the determination of toxicity values for tin compounds is both empirical in nature and unpredictable, and that available data is contradictory is apparent from two papers, one an independent study by Luijten and Klimmer, "A Toxological Evaluation of the Organotin Compounds", Tin Research Institute, Fraser Road, Perivale, Greenford, Middlesex. Revised version of a paper presented at the 18th German tin meeting: "Tin in Chemistry-organic and inroganic tin compounds" of the Zin Informationsburo GmbH, Dusseldorf, on Nov. 15, 1973 at VDI-Hause, Dusseldorf. The other paper is by Pelikan and Cerny, "The Toxic Effects of Some Di- and Mono-n-octyltin Compounds on White Mice", Arch. Toxikol. 26, (3) 196-202 (1970).
The study by Luijten et al. shows that for n-octyltin chlorides toxicity, as measured by LD.sub.50, is most pronounced for the mono-n-octyl compound, less for the di-n-octyl compound, and least for the tri-n-octyl compound. This order of toxicity, however, does not follow when n-octyl tin isooctyl mercaptoacetates are compared; in this case the tri-octyl species is again, unexpectedly, the least toxic, followed by the mono-n-octyl compound. The di-n-octyl compound is the most toxic. However, Pelikan et al. show that when n-octyltin 2-ethylhexyl mercaptoacetate LD.sub.50 values are compared for white mice, the mono-n-octyl species is more toxic than the di-n-octyl compound.
For a material to qualify as a food grade additive it must not present any toxicity hazard at the highest concentration level at which it will be present in the food packaging material. The criteria for determining whether a given ingredient in a plastic material constitutes a toxicity hazard have been defined by the United States Food and Drug Administration (FDA). These criteria are set forth in a comprehensive article that appeared in the October, 1955 issue of the Food Drug Cosmetic Law Journal, and can be summarized as follows:
(1) An ingredient of a plastic material which is not extracted by a foodstuff with which it is in contact does not constitute a hazard.
(2) If a material is found in a food as a result of contact with a plastic, that material may constitue a toxic hazard if it is toxic in the biological sense, i.e. if it causes either an acute or chronic injurious effect by oral ingestion, inhalation, or absorption through the skin, in animals or humans. If no such effect can be shown the material does not constitute a hazard.
(3) Acute toxic levels are unlikely ever to be realized in practice. It is, however, possible that injurious effects may be produced by repeated small doses of a material extracted from a plastic and therefore it is chornic toxicity which should be used for the purpose of assessing the hazard.
(4) The toxic hazard of an ingredient of a plastic material is a function both of its chronic toxicity and of its extractability from the plastic material under service conditions.
(5) For the purpose of assessment of the hazard, extractability tests must be carried out using the foodstuffs themselves or a range of representative extractants under conditions which simulate the most severe conditions likely to be encountered in practice. The results of these tests must then be combined with the data on the chronic toxicities of the ingredients of the plastic as expressed by their Toxicity Factors to give the Toxicity Quotient, which is the measure of the hazard.
To have a composition approved by the FDA and other national health authorities as a food-grade additive, an applicant must submit data from feeding studies on laboratory animals demonstrating that daily consumption of the candidate composition over an extended period of time at concentration levels above those that would be expected based on extractability of the composition from the packaging material does not noticeably impair the health of the animals or result in a significant accumulation of test compound in the blood, bones and internal organs. Since the prior art considers mono-n-octyltin compounds to be undesirable contaminants in a food grade di-n-octyltin stabilizer composition, and limits the maximum concentration thereof to 5% by weight, there is no incentive provided to undertake the extensive feeding and extraction studies required to determine whether the mono-n-octyl compounds would be useful as food-grade stabilizers.
It was surprising to discover that mono-n-octyltin- S, S',S"-tris(isooctyl mercaptoacetate) is actually less toxic than the corresponding di-n-octyltin compound already approved by the FDA as a food grade stabilizer, and that the extractability values for the two compounds are equivalent.